1. Field of the Invention
Aspects of the present invention relate to an electrophotographic image forming apparatus such as a printer, copier and facsimile. More particularly, aspects of the present invention relate to an image forming apparatus having an enhanced sensor configuration to prevent an overheating of heating members.
2. Description of the Related Art
An image fixing apparatus of an electrophotographic image forming apparatus such as a printer, a copier and a facsimile comprises a heat roller containing a heating source, and a press roller installed adjacent to the heat roller that contacts the heat roller with a predetermined pressurizing force. That is, the image fixing apparatus is configured such that a recording medium having an unfixed toner image thereon is passed through a nip between the heat roller and press roller, thereby fixing the toner image on the recording medium with heat and pressure.
To prevent an overheating of the heat roller, the image fixing apparatus includes a thermostat installed, for example, near to the middle of the heat roller in the shaft direction. The thermostat stops an operation of the heating source by shutting off the power supply thereto when the heat roller is overheated.
An image fixing apparatus having the above-described thermostat has been disclosed in Japanese Patent Laid-Open Publication No. 2004-77345. In the image fixing apparatus disclosed in Japanese Patent Laid-Open Publication No. 2004-77345, a substance of low thermal conductivity is sandwiched between a first bimetal and a second bimetal so that the first bimetal and second bimetal bend together in the same direction in response to temperature changes to prevent a malfunction due to effects of overheating.
FIG. 1 is a schematic view showing a conventional image fixing apparatus. With reference to FIG. 1, the image fixing apparatus 1 includes a heat roller 15a embedding a heating source 15c such as a heat lamp and a press roller 15b. The heat roller 15a and press roller 15b tightly contact with each other due to an elastic force of a spring 15f and act to heat-compress a recording medium P passing between the two. A controller (not shown) controls an operation of the heating source 15c to maintain the surface temperature of the heat roller 15a, and permits the recording medium P to pass through a fixing nip N3 between the heat roller 15a having a constant temperature and press roller 15b, so as to fix a transferred image onto the recording medium P. A thermostat 15e installed near to the heat roller 15a shuts off the power supply to the heating source when the heating source 15c is overheated.
In an arrangement of the thermostat 15e, it is necessary to consider the width of a recording medium P passing through the fixing nip N3. That is, the surface temperature of a portion of the heat roller 15a within the width of a recording medium P may lower by about 10° C. than that of some other portion, outside of the width of the recording medium P, not contacting the recording medium P. This is due to the fact that, when recording medium P passes through the fixing nip N3 in succession, heat in the middle of the heat roller 15a is discharged through the passing recording medium R Conversely, heat at ends of the heat roller 15a may not be discharged, resulting in temperature differences.
Accordingly, it is necessary to arrange a thermostat corresponding to a temperature distribution pattern in the heat roller for more effective overheat prevention.